The aerospace industry requires structural metals and alloys that provide maximum strength with minimum weight. Traditionally, these roles have been fulfilled by aluminum, titanium, and alloys thereof. However, as the performance demands of the industry have increased, previously known aluminum and titanium alloys have been pushed to the limits of their usefulness.
The operation of high performance rocket propulsion systems are particularly demanding on metallic components. Extruded and forged parts such as fuel turbopump impellers and other rotational components require high strength and low density, but also require adequate ductility and toughness. Furthermore, because the rotational components of liquid-fueled rocket engines are exposed to cryogenic liquids at very high pressure and low temperature, the rotational components must retain their high strength and ductility in an extremely cold environment.
In the past, high performance aluminum alloy components, such as those used in rocket propulsion systems, were strengthened through precipitation heat treatment, resulting in tensile strengths of up to 80 ksi. The heat treated aluminum parts remain adequate for most modem day propulsion systems but fall short of meeting the demands of today's high-performance rocket engines and other similarly demanding propulsion systems. The components formed by precipitation heat treatment are not particularly suited for use in extremely cold environments such as those temperatures found in liquid fuel rocket engines. Further, heat treatment introduces residual stress and distortion in the metallic components, which is particularly troublesome in thin-walled or high-precision components.
The technology of dispersion strengthening has provided aluminum alloys having strength and durability beyond that provided by precipitation heat treatment. The dispersion strengthened aluminum alloys are metallic aluminum alloys having a second phase material of fine particles, dispersed within the metal in a substantially homogenous dispersion. The second phase particles are typically oxides of the aluminum found within the alloy and may also be nitrides, borides, oxy-nitrides, or carbo-nitrides. The dispersion strengthened aluminum alloys exhibit improved physical properties over and above those of heat treated aluminum, including increased mechanical strength and an increase in the temperature at which the mechanical strength of the alloy begins to fade. Components constructed of dispersion-strengthened aluminum, sometimes known as sintered aluminum products (S.A.P.), have high levels of hardness and tensile strength and maintain those mechanical properties at higher temperatures than comparable aluminum alloys which are not dispersion-strengthened.
The most successful dispersion strengthened alloys have been produced by dispersing metal oxides within metal alloys through cryogenic milling. The cryogenic milling, which usually takes place in liquid nitrogen or a similar chilled atmosphere, provides an ultra-fine dispersion of oxide particles within the alloy and also increases the strain energy that is stored within the alloy, resulting in reduced grain size upon reheating of the metal. The ultra-fine dispersion of oxides and reduced grain size leads to an alloy of relatively high strength, particularly at high temperatures.
The use of dispersion strengthened aluminum alloys in propulsion systems is well studied, and several variations of the dispersion strengthened alloys and methods of producing the alloys are available. For instance, U.S. Pat. No. 3,740,210 to Bomford, et al. discloses the milling of aluminum and aluminum oxide powders in a ball mill with asymmetric organic compounds acting as surfactant agents. The surfactant acts to retard the welding of aluminum to itself within the ball mill, thus allowing the comminuted aluminum and aluminum oxide to be mutually interdispersed in the composite powder. Reduction of the metallic welding also prevents the ball mill from being frozen by agglomerated metal welded between the balls and inner walls of the mill. The favorable intermingling of the aluminum and aluminum oxides provides a composite alloy powder having well dispersed oxides which leads to a high strength, high temperature alloy product.
U.S. Pat. No. 4,818,481 to Luton, et al. discloses the use of cryomilling to disperse a second phase within an aluminum alloy. Luton '481 explains that the repeated fracture and cold-welding of metal powder involved in ball milling causes strain energy to be stored within the milled particles. Recrystallization occurs with longer milling times, resulting in decreased grain size over that of the starting powders. The decreased grain size corresponds to a better dispersed secondary phase within the alloy which, in turn, results in improved mechanical properties in the finished product. Although considerable research has occurred regarding different types of oxide dispersions and methods by which oxide, nitride, and other precipitates are dispersed within aluminum alloys, the improvement in the strength of the dispersion strengthened alloys over the heat treated alloys of the past is fairly modest. Furthermore, the dispersion-strengthened aluminum alloys are designed for use at high temperatures, and are not particularly suited for use in extremely low temperature environments.
Modern, advanced liquid fuel rocket motors require pump components of extremely high strength which are capable of maintaining strength and ductility at extremely low temperatures, due to the use of liquid hydrogen as a rocket fuel. The aluminum components must be capable of continual high-speed operation, typically below −300° F. Existing heat treated and dispersion strengthened aluminum alloys are unable to meet the demands of the next generation of rocket motors and their high stress, extremely low temperature environments.
What are needed are improved aluminum alloys which are not based upon heat treating techniques or dispersion strengthening techniques of the past, and which are capable of withstanding the extremely low temperatures and extreme mechanical stresses inherent in high-performance rocket propulsion systems. What is further needed is a manner of preparing the improved alloys. What is still further needed is a manner of extruding and forging components from the improved alloys in order to obtain products exhibiting extremely high strength and extremely low temperatures.